The present invention relates generally to methods for making tissue products. More particularly, the invention concerns methods for making an uncreped tissue on a modified conventional wet-pressing machine.
In the art of tissue making, large steam-filled cylinders known as Yankee dryers are commonly used to dry a tissue web that is pressed onto the dryer cylinder surface while the tissue web is still wet. In conventional tissue making, the wet paper web is firmly pressed against the surface of the Yankee dryer. The compression of the wet web against the drum provides intimate contact for rapid heat transfer into the web. As the web dries, adhesive bonds form between the surface of the Yankee dryer and the tissue web, often promoted by sprayed-on adhesive applied before the point of contact between the wet web and the dryer surface. The adhesive bonds are broken when the flat, dry web is scraped off the dryer surface by a creping blade, which imparts a fine, soft texture to the web, increases bulk, and breaks many fiber bonds for improved softness and reduced stiffness.
Traditional creping suffers from several drawbacks. Because the sheet is pressed flat against the Yankee, the hydrogen bonds that develop as the web dries are formed between the fibers in a flat, dense state. Although creping imparts many kinks and deformations in the fibers and adds bulk, when the creped sheet is wetted, the kinks and deformations relax as the fibers swell. As a result, the web tends to return to the flat state set when the hydrogen bonds were formed. Thus, a creped sheet tends to collapse in thickness and expand laterally in the machine direction upon wetting, often becoming wrinkled in the process if some parts of the laterally expanding web are restrained, still dry, or held against another surface by surface tension forces.
Further, creping limits the texture and bulk that can be imparted to the web. Relatively little can be done with the conventional operation of Yankees to produce a highly textured web such as the throughdried webs that are produced on textured throughdrying fabrics. The flat, dense structure of the web upon the Yankee sharply limits what can be achieved in terms of the subsequent structure of the product coming off the Yankee.
Another drawback of traditional creping is that the doctor blades used to effect creping on papermaking machines are subject to wear due to contact with the surface of the rotating cylinder. As wear progresses, the effectiveness of the doctor blade is diminished, which leads to progressively more variability in the tissue properties. Creping blades are commonly replaced after a product property of particular importance, such as stretch, bulk, or machine direction tensile strength, has changed from predetermined target levels. Changing creping blades requires considerable down-time and slows production.
The foregoing drawbacks of traditional creping may be avoided by producing an uncreped throughdried tissue web. Such webs may be produced with a bulky three-dimensional structure rather than being flat and dense, thereby providing good wet resiliency. It is known, however, that uncreped tissue often tends to be stiff and lacks the softness of creped products. Additionally, throughdried webs sometimes suffer from pinholes in the web due to the flow of air through the web to achieve full dryness. Moreover, most of the world's paper machines use conventional Yankee dryers and tissue manufacturers are reluctant to accept the high cost of adding throughdrying technology or the higher operating costs associated with throughdrying.
Prior attempts to make an uncreped sheet on a drum dryer or Yankee have included wrapping the sheet around the dryer. For example, cylinder dryers have long been used for heavier grades of paper. In conventional cylinder drying, the paper web is carried by dryer fabrics which wrap the cylinder dryer to provide good contact and prevent sheet flutter. Unfortunately, such wrapping configurations are not practical for converting a modern creped tissue machine into an uncreped tissue machine. Typical creped tissue machines employ a Yankee dryer with a heated hood in which high velocity, high temperature air is used to dry the web at rates well above those possible with conventional cylinder dryers. Most dryer fabrics would deteriorate rapidly under the high temperatures of a dryer hood, and they would interfere with heat transfer to the web. Further, the design of a conventional Yankee hood does not allow an endless loop of fabric to wrap the web through the dryer hood, without prohibitively expensive modifications to the equipment.
Therefore, there is a need for a method for making an uncreped tissue having a three-dimensional structure and offering good wet resilience, high softness and flexibility using a conventional papermaking machine including a Yankee dryer and drying hood. More particularly, there is a need for an adhesion control system which adequately adheres the web to the dryer surface to promote conductive heat transfer and resist blowing forces, while being bound loosely enough to allow the web to be pulled off the dryer surface in uncreped mode without damage to the web.